The invention primarily concerns an inductive coupling circuit. Furthermore, the invention relates to a telecommunication method in sheathed cables of an electric current distribution network.
The creation of new fast access possibilities to the POTS (plain old telephone system) for telecommunication in the local network area—also called local loop or last mile—is currently the subject of intensive development. After abolishment of the monopoly on telecommunication, wide range, intensive competition has started, which has not yet had major effect in the local network area. The main reason for this is that the former monopolists are still the owners of the distribution networks. Alternatives in form of microwave wireless networks or cable TV networks are currently under development. However, due to their high expense they cannot provide a global alternative to existing telecommunication networks in the foreseeable future. On the other hand, electric energy distribution networks certainly have this potential—as verified by numerous studies and field tests. They have a high coverage and are by far more branched than any existing telecommunications network, for they do not only reach every house, but directly every consumer of electric energy and every socket.
Due to these excellent prospects, a new industrial branch has developed under the term “Power Line Communications (PLC)”, as it is called in Europe, or “Broadband over Powerline (BPL)”, as it is called in the USA, which aims at the technical realization of the new potentialities and the provision of the related services. Power Line communication systems are information transmission systems, which distribute information over the power cable medium. In this context, the link between transformer station in the low voltage (LV) grid and the installation in the customer site is particularly important. In most cases, the topology of this network section has a point-to-multipoint structure, with the transformer station as the node. A transceiver unit can, for example, be placed at this node which separately feeds high-frequency signals up to 30 MHz to the outgoing cables and superposes them upon the 50 Hz power current. Further transceiver units can be placed at the customer installation at the interface between LV distribution grid of the utility company and the home installation network of the customer system. Here, high-frequency signals are also fed into and received from the energy cable. The superposition of the 50 Hz power current with high-frequency data signal currents generally involves problems of electromagnetic compatibility (EMC).
As can be seen from extensive studies, different EMV measurements can be provided which reduce the interference from conduction-bound and introduced disturbers. For example, the reduction of conduction-bound interference can be realized with decoupling filters where all parasitic conduction-bound propagation paths of the high-frequency signal are suppressed, as far as possible. The decoupling filters are each deployed at the ends of the link at the transformer station and the house connection.
At the transformer station, the filters prevent propagation of the HF signal to the bus bar and to the parallel branches. Thus, on the one hand, the HF emission at the bus bar is reduced as the interference voltage level at the bus bar is attenuated. Moreover, the conduction-bound superposition of HF signals to parallel branches is prevented which allows utilization of the same frequencies on parallel branches.
The decoupling filter is also used at the house connection as a barrier for HF signals. The decoupling filters prevent unwanted intrusion of the HF signal into the home installation network of the customer system and thus effect a separation of energy supply and high-frequency signals. The utility companies usually use a main supply cable with 4-conductor system for the link between transformer station and house connection on the LV distribution grid. That means that the N conductor (return current conductor of the operating circuit) and the PE conductor (protective earth) are combined in one conductor, the PEN conductor. This kind of network type is called TN-C network (Terre-Neutre-Combiné) in contrast to the TN-S network (Terre-Neutre-Separé), where PE- and N-conductor are conducted separately and are only connected together at a single point. Both transformer station and house connection are grounded in the TN-C network. Hence, the return current of a power consumer can travel both over the N-conductor and over the earth.
Clearly, the same also applies for the high-frequency signal of the data transmission. This spatial separation of outgoing conductor and return conductor causes the generation of high magnetic fields that influence adjacent electric circuits by induction and thus induce interference voltages. In the same way, interference voltages from external sources can also superpose the operating voltage and the data signal. Beside the induction by external magnetic fields, a common mode voltage caused by different ground potentials drives a common mode current through the supply line and data lines, respectively. This common mode current causes a decrease in voltage at the impedances of the line and the consumer, and thus represents another interference source.
Single-phase coupling for data transmission by means of a coupling capacitor (galvanic isolation from the power grid and filtering of low-frequency interferences) and a respective discharging resistor is known for example from EP 0 684 681 A1. Together with a HF transformer, this physical network connection effects a galvanic isolation from the power grid, the filtering of low-frequency interferences by a high-pass effect and an adaptation of the network impedance to the input impedance of a subsequent electronic unit with a multi-stage filtering (passive band pass filters) and controlled gain (control circuit with envelope detector, PI controller, proportional element and non-inverting AC amplifier). Furthermore, a diode of the suppressor type can be connected in the coupling module in parallel to the secondary winding of the HF transformer, which suppresses high-voltage spike.
Furthermore, DE 197 54 800 A1 discloses a gateway unit with which the transmit voltage, which is coupled to the network, is independent of the phase angle of the network impedance (and to a large extent also independent of the absolute value of the network impedance), wherein the galvanic insulation and the receiver sensitivity of the gateway unit and the connected devices are not influenced to obtain an improved suppression of interference signals outside the used frequency band of the HF signals. In particular, in the gateway unit for an installation bus system with information transmission by Power Line there is a coupling circuit with an HF transformer, wherein at least a decoupling capacitor and a compensational inductive are connected in series with the primary winding, and this series connection can be connected to a low-voltage grid. The inductive coupler is dimensioned such in a manner that its impedance compensates the impedance of a decoupling capacitor by means of a series resonance for the frequency of a single used HF carrier signal or for the average of frequencies of HF carrier signals, in case several HF carrier signals are used, wherein the inductive coupler is also dimensioned with respect to leakage inductances of the HF transformer. Furthermore, a protection resistor can be added to the series connection of decoupling capacitor, inductor and primary winding. Finally, for the case that a transmit amplifier is connected to the secondary winding of the transformer, which contains an decoupling capacitor or which is connected DC-free over such a capacitor, an additional compensation is performed on the secondary side by means of a secondary side inductor. This inductor on the secondary side is thereby connected in series with the secondary winding of the transformer and compensates the impedance of the decoupling capacitor for the frequency of the HF carrier signal or for the average of the HF carrier signal frequencies. This type of pre-compensation allows the system to work as well and reach a similar coverage in strong inductive grids as in slightly inductive or even capacitive grids.
Furthermore, DE 199 07 095 C1 introduces a coupling circuit for a data transmission unit to one phase of an electric energy distribution grid which comprises a series connection of a coupling capacitor and a first surge protector, which is located between the phase and a reference potential of the electric energy distribution grid, and a drainage coil which is connected in parallel to the first surge protector and to which a data transmission unit can be coupled. In order to specify a coupling circuit and a coupling unit that contains it, which decouples itself in case of an internal error from the medium-voltage grid in such a way that its operation is not affected, a secondary fuse is provided in the series connection of the phase whose electrical parameters are such that it is triggered neither by a current of the data transmission unit nor by excess voltage of the energy distribution grid. The electrical parameters of the secondary fuse and the first surge protector are matched in such a manner that, in case of alloying of the coupling capacitor, the secondary fuse is already triggered by a rising short-circuit current, quickly and safely by the surge protector. The coupling unit comprises an insulating component that encloses the coupling circuit like a housing, particularly in a shape that is comparable to a pin insulator, in such a way that a first connection of the series connection to a phase of the electric energy distribution grid, a second connection of the series connection to the reference potential and a third connection of the drainage coil to a data transmission unit are accessible from outside.
Furthermore, data transmission on power lines both on low-voltage and medium-voltage lines by means of capacitive coupling is known for example from Proceedings of International Symposium on Power Line Communication and its Application 2000. The respective coverage bypasses numerous transformer stations and partly reaches more than 10 km. The disadvantage of this coupling is the direct connection to the conductor, the respective required withstanding voltage, the space required for installation and the necessity of switching the system off for installation. In very compact transformer stations that are supplied from underground cables, mostly new ones, the installation of a capacitive coupling unit is not always possible.
Furthermore, beside capacitive de-/coupling modules inductive de-/coupling modules for the medium-voltage grid are also known, e.g. from the technical manual DCS3000 of the company Siemens AG, release A1.0/02.2002 pages 1-63 to 1-65, which are simply installed over the cable. The inductive coupling unit consists of a two-part ferrite ring core with an assembly clamp, which is put around the energy cable used for data transmission. Coupling to the transceiver station is done by six windings of a conductor that are wrapped around the ferrite ring core. This requires only little space and can be done during operation of the system, whereby the earth serves as the return path for the data signal, for both capacitive as well as inductive coupling. An inductive coupling is based on the principle that a ring-shaped magnetic field is generated by means of this coupling module. A plane is therefore defined within this ring. If a conductor crosses this plane a current is induced into this conductor. If this current cannot flow without restraint, it generates a voltage that again effects a current flow in the opposite direction and compensates for the induced current. For coupling, the effect is used that a current flow in a conductor generates a ring-shaped magnetic field that can be captured and evaluated from the decoupling module. As coupling out and coupling in can be done with the same device, this device would be regarded as a unit and called de-/coupler. Since coupling is effected into the shield and into the conductor and since the shield is grounded on both ends, the current will be induced directly into the shield. The transmission is only possible between two neighbored transformer substations if the shield is not earthed by an earth sleeve or by a lead mantle of an older cable between these transformer substations. For comparison with a transmission system with capacitive couplers this would afford more transmitter/receivers and the time of transit of the message increases considerably. If a transmitter/receiver is defective, the transmission line is interrupted. For some applications, which are critical with regard to real time capability or security requirements, this is not acceptable.
WO 03/036932 discloses a similar inductive coupler with a least one inductive clamp coupler, which clamps the (medium voltage) power cable used for data transmission. As not to disconnect the power cable during assembly, for example a two-part induction coil or an induction coil with a slit and flexible ring core could be used. The inductive coupling and the power supply to a transmitter/receiver with low voltage level could be done by a wireless coupling device, for example by infrared transmission, wherein either an infrared transmitter or an infrared receiver are arranged in the housing (also with two inductive coils). Alternatively, power supply by solar cells and rechargeable batteries are described.
Furthermore, WO 02/054605 discloses an inductive coupler with an RF-transformer, whereas the power cable will be separated and the RF-Transformer loops in at least one of several neutral conductors such that the primary side level lies between neutral conductor and earth potential. The secondary side is connected to a data transmission device, for example, a modem, whereas the other neutral conductors are used for transmission in the reverse direction.
WO 02/080396 discloses a further development of this inductive coupler in the form of a two-part magnetic coil, which clamps the power cable, whereas the secondary winding is wound around the core, and a part of the power cable which passes through the core was used as primary winding. Finally, a capacitive coupler can be arranged at the secondary side of the transformer.
The above discussion of prior art acknowledges inductive coupling units of different design that mostly contain a transformer. An installation during operation of a 10/20 kV power grid is only possible if no contact is necessary in the security area, which is in most cases additionally protected by constructive measures. Hence, a capacitive coupling module as well as the installation of an inductive coupling module directly over a conductor are not possible.
Due to safety reasons, the shields of underground cables between two transformer stations are connected on both ends with earth or a compensational potential. Sometimes, there is a separate cable with its own shield for each phase (three phases), and sometimes, the three phases are combined in a single cable, surrounded by a common shield (as shown in FIG. 1 and FIG. 3). According to the laws of physics, the current takes the path of least resistance. Considering two electric circuits crossing the plane defined by the inductive input coupling, the major part of the current is induced into the electric circuit with the lower resistance.
For the current installation of inductive coupling modules, the electric circuit with the very low resistance is the shield that is grounded on both ends. Thus, the current is induced into the shield. If the shield is grounded between two neighboring transformer stations, as for example in a earth sleeve or the cable has a lead shield isolated with bitumen which has contact to ground water, the electric circuit is closed early and does not reach the neighboring transformer station. Electromagnetic interferences in the surroundings are collected by the shields of the cables and discharged to earth. These compensational currents have an order of magnitude that is absolutely relevant for communication and are captured during decoupling as an interference signal. According to this, an inductive coupling unit is missing in practice which allows a direct induction of the current into the conductor and which prevents capture of the compensational currents on the shield during decoupling. This is especially important because the industry manufacturing medium-voltage or high-voltage systems can be considered as a very progressive industry which quickly picks up improvements and simplifications and puts them into practice.
In contrast to the known inductive coupling devices and methods for information transmission in electric energy distribution networks with shielded energy cables, it is the underlying purpose of the present invention to provide such an inductive coupling device and a method which achieves the coverage provided by capacitive de-/coupler.